VOAs of various designs are widely used in the local and long distance telephone networks. Applications of these devices include, e.g., light filters, switches, splitters, combiners/couplers of light signals, light multiplexers, etc. It is expected that thermally compensated, fiber coupled, VOA's will become vital for the development of high performance telecommunication systems. The ability to design viable and reliable packages, which might be subjected to various thermal and mechanical loads during 20-25 years of their projected service life, is of practical importance.
Optical systems utilizing fiber coupled variable taps are already known (see, e.g., Miller et al. U.S. Pat. No. 5,146,519 entitled "Rotary Variable Optical Tap"; Keck et al. U.S. Pat. No. 5,353,363 entitled "Optical Fiber Bendable Coupler/Switch Device"). The variable attenuation of the light signal is achieved by bending or rotational (torsional) loading of a bi-conical coupler comprising a pair of optical fibers fused together at a narrowed region. Typically, the coupler is bent in the narrowed region, whereby a coupling ratio can be selected. Structurally, fiber coupled variable optical attenuating (FC-VOA) packages are multi-material assemblies in which the material interactions, their sizes and configurations, and the loads, whether thermally induced or mechanical, are as important as the performance characteristics of the employed materials. The thermal contraction mismatch of the fused silica glass used in the biconical coupler and fibers relative to the enclosure materials (metal alloys or packaging plastics), including adhesives, results in thermally induced stresses that affect the light attenuation and control thereof. In order to minimize the adverse effect of thermally induced stresses in the fused coupler caused by the thermal contraction mismatch between the fused glass coupler, the loading mechanism, and the enclosure, the fiber may be mechanically tuned by alignment techniques. The tuning, however, results in a new temperature dependent change in the light attenuation caused by the combined action of the thermal mismatch and the tuning induced misalignments. The loading devices used in FC-VOA packages tend to not be fully thermally compensated and therefore produce additional mismatch stresses when assembled in the protective enclosure. These packages are also sensitive to mechanical shock and vibrations and, therefore, reproducibility of the coupling ratio varies over time. In addition, FC-VOA packages containing loading devices providing variable bending or rotation are very expensive to manufacture.
It is desirable to obtain a high accuracy, totally thermally self-compensated fiber coupled variable optical attenuating system that can be easy-tunable over large deflection and temperature ranges, is relatively inexpensive, which is made from commercially available and easily machinable materials, and which will be reliable in operation. In addition to this, the enclosure design should be adequate not only to mechanically protect the brittle glass components but also to compensate for and minimize the possible thermally induced shift in the optical attenuation.